Ball dimensional variability

[ToddF]

We just wanted to pass on a bit of news.

Last night, as we began prototyping shooting and pickup mechanisms, we realized that we had been accepting the specs and CD for the dimensions of the ball. As a double check, we actually measured our ball. We did this by placing it against a wall, with a vertical surface touching the other side of the ball, measuring its diameter caliper style.

The first thing we noticed is that after 4 days the ball is no longer round. Rather, it is "lobed". Rotating the ball to measure the max diameter across the lobes gives a max dimension of 25.75", a good deal larger than the ~24" specified in the game manual.

We wanted to get this out there to make teams aware that if they design mechanisms for a 24" diameter ball, the actual balls may become jammed.

[Anupam Goli]

Todd,

How much of wear (if any) does the ball have, and how much of use has it gone through? We'll have to check our own ball's dimension and see if it's matching the change in shape that you seem to be getting. Perhaps the colder temperature is affecting the air pressure of your ball?

[wilsonmw04]

Originally Posted by Anupam Goli

Todd,

How much of wear (if any) does the ball have, and how much of use has it gone through? We'll have to check our own ball's dimension and see if it's matching the change in shape that you seem to be getting. Perhaps the colder temperature is affecting the air pressure of your ball?

If the temp were the case, it would make it smaller not larger.
My guess is that it has more to do with the cover than the ball. Variability is going to be something we are going to have to deal with. 1.5" is not that big of a concern. It's better than the basketballs in Rebound Rumble.

[ToddF]

Very little wear, and very little use. We've tossed it around and bounced it by hand a bit. We also took it outside to test some crude shooter schemes (see-saw using a 2x4). After a few minutes in the cold (~20 deg F), it had shrunk noticeably, with the cover becoming wrinkled. Bringing it back inside restored the ball to size.

[Rishabhgadi]

Originally Posted by wilsonmw04

If the temp were the case, it would make it smaller not larger.
My guess is that it has more to do with the cover than the ball. Variability is going to be something we are going to have to deal with. 1.5" is not that big of a concern. It's better than the basketballs in Rebound Rumble.

After the experiance with the Basketballs I beleive everyone is prepared for this.

[MechEng83]

Originally Posted by Rishabhgadi

After the experiance with the Basketballs I beleive everyone is prepared for this.

Except for all the rookies from 2013 and 2014, whether it be new teams, or new students/mentors.

[Anupam Goli]

Originally Posted by wilsonmw04

If the temp were the case, it would make it smaller not larger.

Whoops, got it backwards. Sorry about that!

Originally Posted by wilsonmw04

My guess is that it has more to do with the cover than the ball. Variability is going to be something we are going to have to deal with. 1.5" is not that big of a concern. It's better than the basketballs in Rebound Rumble.

Very little wear, and very little use. We've tossed it around and bounced it by hand a bit. We also took it outside to test some crude shooter schemes (see-saw using a 2x4). After a few minutes in the cold (~20 deg F), it had shrunk noticeably, with the cover becoming wrinkled. Bringing it back inside restored the ball to size.

I think wilsonmw04 is correct in suspecting that the change in ball shape may be due to the cover of the ball and the shrinking/expanding it went through. As the ball's volume changes, the cover gets more worn and stretched.

[RaxusPrime]

Do you think it might be a major factor during competition then?

[wilsonmw04]

I don't think so, unless you are designing something that needs to be snug to the ball. Since is gives a bit when force is applied, I don't see it being an issue.

[kws4000]

Do we happen to have a mass on this ball? Can't be much more than a pound and a half, right?

[M.O'Reilly]

I am surprised that your ball is deformed, and I surmise it was manufactured that way. The only way I could imagine that the ball would get deformed is from someone sitting on it. Again, normal exercise balls are meant to bear weight, so my guess is it was manufactured this way, and we can expect to have to deal with heterogeneity at competitions.

[Henrique Schmit]

Originally Posted by kws4000

Do we happen to have a mass on this ball? Can't be much more than a pound and a half, right?

I've heard that one of our mentors calculated the ball's mass as more than 1kg, i think it was 1,3kg.

[kws4000]

Originally Posted by Henrique Schmit

I've heard that one of our mentors calculated the ball's mass as more than 1kg, i think it was 1,3kg.

So that works out to 2.86 pounds. Thanks!

[TheOtherGuy]

Originally Posted by Henrique Schmit

I've heard that one of our mentors calculated the ball's mass as more than 1kg, i think it was 1,3kg.

Does this include the mass of the air when filled?

[JamesCH95]

Our ball measured 25in +/- 3 in as measured over several different diameters.

The fact that yours is 25.75in OD is somewhat disconcerting...